This invention relates generally to the optical arts of holography and diffraction gratings, and more particularly, to a special forms of these optical devices for use in authenticating documents and other items as genuine.
There is a wide-spread need to authenticate written information and articles in order to prevent both the counterfeiting of the article and an alteration of a genuine article. Examples of such articles include credit cards, identification cards, driver's licenses, transit passes, passports, stock certificates, and the like. Holograms are currently being extensively used on credit cards and transit passes, for example. An image having observable three-dimensional characteristics is viewable in light reflected from such a hologram.
Because holograms are difficult to make and require a high level of technical sophistication, the difficulty of making a counterfeit credit card, or other article to which a hologram is attached, is increased. A master hologram is made in an optical laboratory with coherent light. The resulting master hologram is capable of reconstructing an image of the object from which it is made. Low-cost replicas are then made from that master. Several specific replication techniques are known, the most common currently being to make the master in a manner that the object information is stored in a surface relief pattern. This surface relief pattern is then used to make replicas by an embossing or casting operation. A layer of reflective material is applied to each replica surface relief pattern so that the image may be viewed in light reflected from that pattern. The holograms are then firmly attached to the credit card or other article to be authenticated. The hologram is also constructed of suitable materials so that an attempt to remove the hologram from the article will destroy it, thereby eliminating the possibility of a counterfeiter transferring a hologram from a genuine article and onto a counterfeit article.
An authenticating hologram of this type cannot cover printing, photographs and the like carried by the article to be authenticated when that information wants to be viewed. This is because the reflective layer of the hologram is substantially opaque so that any visual information on the article in the area to which it is attached is covered by the hologram. Some credit cards are authenticating some of the raised alpha-numeric information on them by covering at least a few of the characters with the hologram. But any printing, photographs or the like on a flat surface under the hologram is not viewable.
There has been some effort to control the amount of reflective material that is applied to the surface relief hologram so that it will both reflect enough light in order to reconstruct an image recorded in the hologram and at the same time be transparent enough to make visible any printing, photograph or other visual information under the hologram. Besides authenticating the article to which the hologram is attached and the visual information under it, such a hologram has the potential of preventing alteration of that information on an authentic article. It has been found to be very difficult, however, to be able to repeatedly form a reflective layer on the hologram that gives it these advantageous characteristics. Aluminum is usually employed as the reflective material. A proper thickness of that layer can result in the desired partial transparency and partial reflection, but the processes used to date have been very difficult to control in order to provide the proper thickness.
Holograms are also being used to authenticate documents, such as transit passes, that are reissued at frequent periodic intervals, such as monthly. It is often desired that the hologram be changed each month so that transit authorities can easily recognize a valid current transit pass from an expired one. This presently requires making a new optical master hologram for each new pass, an expensive and time consuming process.
Therefore, it is a specific object of the present invention to provide an improved hologram or diffraction grating for attaching to a document or article over visual information that is to be protected, wherein the information can be observed through the hologram and an image reconstructed from the hologram may be observed.
It is another object of the present invention to provide an authentication hologram or diffraction grating, and process for making it, that allows easy modification for monthly transit passes and the like without having to make a new optical master.
It is a more general object of the present invention to provide novel holograms and diffraction gratings for authentication of documents and other articles.